This invention relates to a process and associated apparatus for the low-temperature separation of air in which a first air feedstream is compressed, prepurified, cooled, and at least partially introduced into the high pressure stage of a two-stage rectification system and wherein both gaseous oxygen and gaseous nitrogen are preferably removed from the low pressure stage.
In the separation of air by two-stage rectification, the air feedstream is generally passed into the high pressure stage, where it is preseparated into nitrogen-rich and oxygen-rich fractions which are further rectified in the low pressure stage. In this case, the air feedstream must be compressed to the pressure of the high pressure stage, i.e., to about 5 to 7 bars. Air compressors for this purpose involve high investment and operating costs.
A unit used mainly for the production of oxygen of relatively low purity, for example, lower than 98%, can also be operated so that a part of the air feedstream is passed without preseparation in the high pressure stage directly into the low pressure stage without significantly reducing the oxygen yield. In particular, a process used for recovery of energy from the air feedstream compressed to the pressure of the high pressure column is described in DE-PS No. 28 54 580. In that process, a part of the air feedstream is expanded, after compression, to the pressure level of the low pressure stage and then is fed to the low pressure stage. The refrigeration values recovered during expansion are used for the liquefaction of product gases.
Conversely, in the process of DE-PS No. 28 54 580, the recovery of the compression energy is necessarily incomplete, even with the use of a combination of an expansion turbine and a mechanically coupled compressor. Moreover, the known process lacks flexibility, since the efficient use of expansion turbines is limited to a relatively narrow range of rates of flow. Thus, the amount of air passed directly into the low pressure stage can be changed to only a limited extent during operation.